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I am working on a project and have a bit of a challenge in developing power. I have played with several ideas and still not satisfied with the solution.

Power input: ac ranging from 3 Hz at 2.5 Vrms (start-up) to 80 Hz at 28 Vrms. Yes, both change as the voltage or frequency increase. Source impedance 2 Ohm maximum.Power output: dc, 5V, 50 ma maximum, more like 25 ma continuous.Problem: Operating above 8 Vrms is no issue since have a buck regulator running well. Challenge is at the low input voltage.Current solution: operating at 2.5 Vrms and using a full-bridge rectifier is not a reasonable solution since any diode bridge kills any form of voltage to work with. I have a voltage doubler implemented and then use an isolated sepic to generate the 5 Vdc. Problem is that the capacitors for the sepic is very large and I have a space constrained volume to work with.

Any suggestions appreciated. I have been scratching my head for a few weeks in order to come up with other options. Please post ANY suggestion and we can discuss.